Transubstantiation is kind of in the Bible. Matthew 26:26-28

While they were eating, Jesus took bread, and when he had given thanks, he broke it and gave it to his disciples, saying, “Take and eat; this is my body.”

Then he took a cup, and when he had given thanks, he gave it to them, saying, “Drink from it, all of you. This is my blood of the[a] covenant, which is poured out for many for the forgiveness of sins.

The discussion of transubstantiation is just how literal “my body/blood” is.